One Merge to Rule Them All: From Galaxy Interactions to Black Hole Mergers Using Horizon-AGN

TL;DR

This study uses the Horizon-AGN simulation to analyze galaxy and black hole merger histories, revealing their evolution over cosmic time and providing a framework for future observations and gravitational wave predictions.

Contribution

It introduces a novel method for identifying galaxy pairs using MCC, linking galaxy mergers to black hole coalescences, and offers refined criteria for observational studies.

Findings

Galaxy merger activity increases with stellar mass and redshift.

Black hole merger rate peaks around cosmic noon (z~2-3).

Strong correlation between galaxy and black hole merger histories.

Abstract

Galaxy mergers are fundamental drivers of galaxy evolution and black hole (BH) growth across cosmic time. We use the Horizon-AGN simulation to investigate the fraction of galaxy pairs, the merger fraction, and the galaxy merger rate over a wide range of stellar masses and redshifts. To identify physically connected pairs, we adapt the Matthews Correlation coefficient (MCC) framework, optimizing thresholds in projected distance and redshift difference, and compare our selection to commonly used criteria in the literature. We then connect the derived galaxy merger rates to supermassive BH mergers, tracking the evolution from galaxy interactions to BH coalescences, thereby reconstructing the full merger history. We find that the galaxy pair fraction, merger fraction, characteristic timescale, and merger rate all evolve strongly with both stellar mass and redshift, with higher-mass galaxies and earlier galaxies showing elevated merger activity. BHs exhibit a similar evolutionary trend, with the volume-averaged BH merger rate peaking around cosmic noon ($z\sim2\mbox{--}3$). Our results demonstrate a close correspondence between galaxy and BH cosmic histories. This work provides a comprehensive, simulation-based framework for linking galaxy and BH merger populations, and offers refined selection criteria for future observational studies, for forecasts of gravitational wave detections with LISA, and interpretation of Pulsar Timing Array results.